Lab: Mixing Fixers from Scratch

Purpose

To mix three fixing baths from named salts — a plain hypo bath, an acid hardening bath of the F-5 type, and a diluted rapid ammonium thiosulfate concentrate — measuring the pH of each, doing the hydrate arithmetic that turns one form of a salt into another, and reproducing on twenty millilitres the one reaction the mixing order exists to prevent.

The three litres are the working stock for the capacity experiment and for the break/fix session, so label them as though somebody else will use them — because somebody else will: you, in three weeks.

Learning objectives

Convert between a hydrate and an anhydrous salt and check the answer against a manufacturer’s own alternative; state, for every ingredient in an acid hardening fixer, what it is for and what fails without it; mix a bath in a published order and explain each step in terms of the failure it avoids; measure the pH of a solution you mixed and say what a reading outside the expected range would mean; and label a bottle so that its contents are reconstructable from the label alone.

Prerequisites

How fixer works and fixer formulations and types, which supply every reason this session’s order and quantities have. From Part II, concentration and dilution, measurement and uncertainty and laboratory safety and PPE. From Part III, the pH laboratory — you must be able to calibrate a meter before any pH number here is worth writing down. From Part X, stop bath formulations compared, which owns the acetic-acid dilution arithmetic this page uses without re-deriving.

Safety classification

Level A on the course rubric, and the assessment is worth following because one ingredient decided the design of the session.

What applied. Dilute solutions in gram and hundred-gram quantities. Two solids weighed at the fifteen-gram scale and one at the several-hundred-gram scale, all of them non-volatile. Potassium alum carries no hazard statement in any of the 43 ECHA reports across its two notifications; sodium thiosulfate carries none in 98.6 per cent of its 281 reports. Sodium sulfite does carry H314 and is the most seriously classified solid on the bench, handled as a 15 g weighing and then as a dilute solution. The acid is bought dilute and never handled as the concentrate, exactly as Part X established: glacial acetic acid is H314 and H226 and handling it is Level B work. Nothing is heated above hand-warm. There is no silver anywhere in the session, so there is no silver-bearing waste.

⚠One Level B step: weighing the boric acid

The session’s baseline is Level A, with weighing and adding 7.5 g of boric acid crystals at Level B, under the raised-step rule. The bath is complete F-5: the buffer is retained, and its reproductive-toxicity classification is not lowered by describing the rest of the session as Level A.

Before opening the container, provide the controls in Chemical Safety Card 0991: prevent dust dispersion, use local exhaust or appropriate breathing protection, and wear protective clothing, gloves and eye protection. A tray contains spills; it does not replace inhalation control. Keep the vessel close to the receiving container, close the stock promptly, cover broken skin and wash hands before leaving the work area. Do not eat or drink at the bench.

If you cannot provide those controls, omit the F-5 preparation, not an ingredient: follow the plain-fixer and bought-rapid-fixer comparisons instead. Record that reduced experimental scope. The boric acid entry explains the hazard; the manufacturer’s mixing order places this addition after the acid and before the alum.

What is not a hazard here, and why. Three things a reader might expect to be dangerous are not, in this session, and it is worth naming them because knowing what is absent is half of a hazard assessment.

There is no silver. The word “fixer” carries a reputation earned by spent fixer, the most silver-laden liquid in a darkroom. What you mix today has dissolved nothing, so nothing here belongs to the silver-bearing stream. That changes the moment these baths meet a film, and the experiment page treats them accordingly.

There is no sulfur dioxide in the ordinary run of the session. The reaction that makes it needs acid and thiosulfate meeting without enough sulfite, and the mixing order is designed so that they never do. It happens once, deliberately, on 20 mL, under the ventilation stated below — and the reason the demonstration is worth doing is precisely that the rest of the session avoids it.

The alum is not the chrome alum of the historical formulas. Potassium alum is an aluminium salt with no GHS classification at all. What it does have is a dust: EH40 sets soluble aluminium salts 2 mg/m³ as aluminium over eight hours, which is a control that no hazard statement mentions and the reason the alum is weighed without raising a cloud. Chromium is not in this session; the chromium policy governs where it is.

Hazards

What	Why	Control
Sodium sulfite, solid and in solution	H302, H314, H315, H319 in the aggregated ECHA classification — the most seriously classified solid here	Gloves and eye protection from the moment the tub is opened; weigh over a tray; do not raise dust
Dilute acetic acid at a stated strength	The concentrate is H314 and H226; a 28 % w/w product is a different proposition and the difference is that you bought it dilute	Never buy or handle glacial for this course; eye protection; add acid to solution, never solution to acid
Potassium alum, fine crystals	No GHS classification; EH40 gives soluble aluminium salts 2 mg/m³ over eight hours, which is a dust control	Weigh without raising a cloud; gloves and eye protection
Sodium thiosulfate meeting acid	The supplier’s own safety data sheet says it reacts with acids to form sulfur dioxide and hydrogen sulfide	The mixing order; the 20 mL scale of the demonstration; ventilation; the vessel covered between observations
Sulfur dioxide from the deliberate demonstration	EH40 gives it 0.5 ppm over eight hours and 1 ppm over fifteen minutes, among the tightest gas limits in the course	20 mL of bath and about 1 mL of dilute acid, in a covered vessel, at the far end of the bench from where you are working
Ammonium thiosulfate concentrate	No agreed GHS classification on a thin evidence base; CAMEO records the solution at 60 per cent or less as odourless, and the supplier’s data sheet for the product you bought is the authority	Splash goggles for the undiluted concentrate; gloves; do not raise its pH
A 500 g weighing on a balance sized for 200 g	An overloaded balance reads, and reads wrongly	Check the capacity on the plate before the first weighing, and weigh the bulk salt in portions if you must

Required PPE

Nitrile gloves throughout, at the thickness HSE’s COSHH essentials sheet P1 names as typically acceptable splash protection where the safety data sheet gives nothing more specific — single-use nitrile, about 0.2 mm. Eye protection from the first cap to the last; chemical splash goggles rather than spectacles for two operations, the fixer concentrate before dilution and the sulfite tub while it is open. An apron or overall, which is what ILFORD’s own sheet asks for when handling and mixing any of its chemicals; dried fixer leaves a white bloom that will mark a print it later touches. The glove page records that the permeation guide the course read carries no entry at all for sodium or ammonium thiosulfate, so the glove here is splash protection with no breakthrough time behind it, and it comes off the moment it is splashed.

Ventilation

The control is general ventilation at the standard HSE’s COSHH essentials sheet P1 sets for manual film development: more than five air changes an hour with a through draught — an open window and an open door, or an extractor. That is the standard for the whole session and not only for the demonstration.

For the demonstration specifically, the supplier’s safety data sheet for sodium thiosulfate asks for local exhaust “if there is a release of sulfur dioxide and/or hydrogen sulfide gas”. A home darkroom does not have local exhaust, so the course substitutes the two controls it does have: quantity — 20 mL of bath and about 1 mL of acid, which is the smallest amount in which the milkiness is unmistakable — and containment, the vessel covered with a watch glass or a saucer between observations. The course has not measured the airborne concentration this produces and does not claim it is negligible; it claims that the quantity is small, the room is ventilated, and the observation takes under a minute.

Materials

Water for the baths — deionised, or the tap water you will actually process in, but the same water every time and written on the label, for the reason Part III gives. Three one-litre storage bottles in HDPE or glass with sealing caps, one small bottle for the 1+9 sample, labels, and a pen that survives a splash.

Chemicals

Chemical	Quantity for the session	Form
Sodium thiosulfate pentahydrate	490 g	Coarse crystals; the form every published formula is weighed in
Sodium thiosulfate, anhydrous	alternative to the above, 312 g	Powder or granules; only if that is the grade you own
Sodium sulfite	15 g	Anhydrous. If you hold the heptahydrate crystals, 30 g
Acetic acid	63 g of a 28 % w/w product, or 22 g of an 80 % w/w product, plus about 1 mL for the demonstration	Bought dilute at a strength the label states. Never glacial
Potassium alum	15 g	Fine crystals, which is the form it is sold in because the powder cakes
Ammonium thiosulfate	210 mL of a bought rapid fixer concentrate	Liquid concentrate; the supplier’s own data sheet is the authority for it
Boric acid	7.5 g	Crystals; weighing and addition are the declared Level B step

Equipment

A balance whose capacity covers a 240 g weighing and whose readability is 0.1 g or better — the two are different specifications sold under the same words, as the planner records. A 1 litre graduated cylinder and a 100 mL one, because ILFORD’s own advice is that it is easier and more accurate to measure 100 mL into a 100 mL cylinder than into a 1000 mL one. Two 2 litre beakers or jugs, a 50 mL beaker and a watch glass or saucer for the demonstration, a stirring rod, a thermometer, a pH meter with buffers at 4.01 and 7.00 (or narrow-range papers covering pH 4 to 6, which ILFORD says are sufficient), a funnel, and a scrap of undeveloped film for the clearing-time reference.

Estimated cost

Band £, and it is the cheapest litre-scale chemistry in the course after the stop bath.

The planner carries a dated price for the rapid fixer concentrate, of which this session uses about a fifth of a litre, and since 7 September 2026 it also prices sodium thiosulfate pentahydrate as a raw salt — which is the largest quantity on this page and was the largest hole in its costing. It still records honestly that it could not price the alum, one of the named gaps on the prices page. Buy the hypo in the largest packet you will use inside a couple of years, and check the grade rather than the label: what Kodak’s 1924 primer warned about is dirt, straw and wood dust from careless handling, and calcium thiosulfate, which decomposes far more readily than the sodium salt.

Estimated consumables cost

The cheapest litre-scale chemistry in the course after the stop bath, and better costed than it was: the hypo that is the largest quantity here now carries a dated price, and the alum that is the second still does not.

Consumed	This session	Sourced price	Cost this session
Sodium thiosulfate pentahydrate	490 g, or 312 g as the anhydrous salt	£14.70 per 1 kg of the pentahydrate, checked 7 September 2026	£7.20 for the pentahydrate route
Sodium sulfite, anhydrous	15 g, or 30 g as the heptahydrate	£13.68–£19.98 per 1 kg, anhydrous	£0.21–£0.30
Acetic acid, bought dilute	63 g of a 28 % product, or 22 g of an 80 % one	£11.89 per 1 L of the 80 per cent acid. This row is a mass and the listing is a volume, so no cost follows from it without a density	—
Potassium alum	15 g, fine crystals	None. A named price gap: chrome alum, potassium alum and sodium hydroxide	—
Ammonium thiosulfate rapid fixer concentrate	210 mL of a bought concentrate	£21.05–£25.98 per 1 L of ammonium thiosulfate concentrate, diluted 1+4 for film	£4.42–£5.46
Boric acid	7.5 g	No dated price is recorded for this reagent	—
Storage bottles, labels and a waterproof pen	three 1 L bottles and one small one; the bottles are kept	None. A named price gap: storage bottles in HDPE or glass	—
A scrap of undeveloped film	one corner, the clearing-time reference	A by-product, not a purchase	—
Water, the same water every time	about 3 L	None. distilled-water carries a cost band and no dated figure	—

The priced rows come to £11.83 to £12.96 for one run of this session, at the retail ranges read on 5 and 7 September 2026 and recorded in the laboratory planner. That is a floor, not a total: 5 of the 9 rows carry no dated price, so they are counted as nothing here and are certainly not free. A priced entry is a dated range to plan against, never a quotation.

Buy the hypo in the largest packet you will use inside a couple of years, and check the grade rather than the label. The priced subtotal above covers three of the nine rows, and the hypo is now most of it: a kilogram of the raw salt costs less than a litre of rapid fixer concentrate and goes a great deal further.

Waste streams

Almost nothing, and that is itself the lesson. The three baths are products, not waste. They have dissolved no silver, they are going into labelled bottles, and they will be used. Fixer becomes a waste stream the moment it meets a film, and it is the experiment page that produces one.

What this session does produce:

• The sulfurised demonstration, about 21 mL, which is acid and carries colloidal sulfur and bisulfite. It goes to the acid container under the general chemical waste procedure, whose second step is exactly this decision, and it must never go into a container holding a sulfite or a thiosulfate stream.
• Rinse water off the beakers and cylinders, which carries traces of thiosulfate, sulfite and alum and no silver at all.
• Packaging and wipes, as solids, in their own bag.

Preparation

1. Read the formulations lesson if you have not, because every step below has its reason there and the procedure will not repeat them.
2. Do the arithmetic before you touch a salt, on paper, and check it against the callout below. The commonest error in this session is not a mis-weighing; it is weighing the right number of grams of the wrong hydrate.
3. Calibrate the pH meter at 4.01 and 7.00 and write the slope in the notebook, per the pH laboratory. A meter that has not been calibrated today produces numbers, not measurements.
4. Label the three bottles before they are filled, following the labelling procedure. A bottle labelled after filling is a bottle that was, for some period, unlabelled.
5. Stand the water for the baths where it will reach room temperature, and set aside about 600 mL of it slightly warmer than the working temperature — Reilly’s instruction, for the reason the observations section explains.

🧮The hydrate arithmetic, and a check that Kodak passes and you can too

Kodak Limited’s F-5 gives its two salts twice over, in the crystalline form and the anhydrous. That is a free check on your own arithmetic, and it is worth doing before you weigh anything.

The thiosulfate. The pentahydrate’s relative molecular mass is 248.19 and the anhydrous salt’s is 158.11, so a gram of crystals carries 158.11 ÷ 248.19 = 0.637 g of the anhydrous salt.

240 g × 0.637 = 152.9 g

F-5's 240 g of crystals, as the anhydrous salt

Kodak’s own alternative is 150 g. The handbook is 1.9 per cent below the arithmetic, which is the handbook rounding to a convenient number and is comfortably inside the tolerance of a fixing bath. It is not an error in either direction, and noticing it is the point: a published alternative is a second measurement of the same quantity, and two measurements that agree to two per cent is a check passed.

The sulfite. Sodium sulfite heptahydrate is 252.18 against the anhydrous 126.04, so the crystals are almost exactly half sulfite by mass.

30 g × (126.04 ÷ 252.18) = 15.0 g

F-5's 30 g of crystals, as the anhydrous salt

Kodak’s alternative is 15.0 g. Exact.

The acid. Part X worked this route in full and this page applies it. F-5’s 17.0 mL of glacial acid, at PubChem’s density of 1.049 g/mL and about 99.5 per cent purity, is 17.0 × 1.049 × 0.995 = 17.75 g of acetic acid, which is 0.296 mol. Get that from a product whose label states a strength by weighing, not by measuring volume: 17.75 ÷ 0.28 = 63.4 g of a 28 % w/w product, or 17.75 ÷ 0.80 = 22.2 g of an 80 % one.

One number worth pausing on. Kodak’s SB-1 stop bath is also 17 mL of glacial acetic acid per litre. The acid reserve Kodak put in its stop bath and the acid reserve it put in its fixer are the same reserve, which is a coincidence only if you have not read Part X: both baths are sized against the same arriving alkali.

Procedure

The session, in five movements

1. 1Bath 1 — plain hypo, 1 litreOne salt. Ten minutes, and the thermometer is the instrument.
2. 2Bath 2 — the acid hardening bath, 1 litreFour additions in a published order, each one waited out completely before the next.
3. 3The demonstration — 20 mL, wrong order, on purposeThe one thing in this part you will not forget.
4. 4Bath 3 — the rapid concentrate, 1 litre at 1+4Concentrate into the vessel first, then the water, stirring throughout. The opposite of the acid rule, and for a different reason.
5. 5Measure, label, logpH of all three, the clearing-time reference on a scrap of film, and a label somebody else could rebuild the bath from.

Bath 1: plain hypo, 1 litre at 25 % w/v

1. Put 600 mL of the slightly warmed water into a 2 litre beaker and take its temperature. Write it down.
2. Weigh 250 g of sodium thiosulfate pentahydrate — or 159 g of the anhydrous salt, if that is what you have — and add it in three or four portions, stirring each in before the next.
3. Take the temperature again when it has dissolved, and write that down too. This is a measurement, not a formality; see the observations below.
4. Make up to 1000 mL with water at room temperature and stir.
5. Measure and record the pH. The course holds no published pH for a plain hypo bath, so this reading is yours and there is nothing to check it against; record it as the reference for your own future batches.

Bath 2: the acid hardening bath, 1 litre

Every step here answers a failure named in the formulations lesson. Do them in order and wait out each one.

6. Put 600 mL of the slightly warmed water in the second beaker.
7. Dissolve 240 g of sodium thiosulfate pentahydrate completely. Completely is the operative word: the 1928 primer warns that adding hardener to hypo that has not fully dissolved is apt to precipitate sulfur. If the beaker has gone cold, let it come back before continuing.
8. Add 15 g of anhydrous sodium sulfite (or 30 g of the heptahydrate crystals) and dissolve it completely. Nothing acid goes near this beaker until the sulfite is in solution.
9. Add the acid, weighed as computed above, slowly and with stirring. Pour it into the bath; never pour bath into acid.
10. Level B step: confirm the inhalation and personal-protection controls declared under Safety classification, weigh 7.5 g of boric acid crystals, and dissolve them completely. Close the stock container before continuing. If the controls are unavailable, do not prepare this bath.
11. Dissolve 15 g of potassium alum, last, in a bath that is at room temperature. Add it steadily, stirring hard. If the bath is warm, wait.
12. Make up to 1000 mL, stir, and let it stand for ten minutes before judging its clarity.
13. Measure and record the pH.

The demonstration: 20 mL, in the wrong order, deliberately

14. Take 20 mL of Bath 1 — the plain hypo, which has no sulfite in it — into a 50 mL beaker, at the far end of the bench, with the window open.
15. Add about 1 mL of the dilute acetic acid and stir once.
16. Cover the beaker with a watch glass or a saucer and look at it against a dark background at 30 seconds, 2 minutes and 10 minutes. Write down what you see each time.
17. Leave it covered. It goes to the acid waste container at clean-up and nowhere else.

Bath 3: the rapid concentrate, 1 litre at 1+4

18. Read your own product’s sheet for its dilution. ILFORD’s is 1+4 for all film applications and 1+4 or 1+9 for manual paper work; other makers differ, and Foma’s is 1+5. The number below assumes 1+4.
19. Pour 200 mL of concentrate into the mixing vessel first, then add 800 mL of water gradually while stirring. This is the opposite of the acid rule and it is there for a different reason: ILFORD’s sheet says fixer concentrates “do not readily mix with water”, so the stirring is not optional and a badly mixed bottle is a bath of two strengths.
20. Measure and record the pH, and the specific gravity if you own a hydrometer.
21. Take 10 mL of concentrate into 90 mL of water in the small bottle to make a 1+9 sample, and measure its pH and specific gravity too.

Measure, label and log

22. Take a scrap of undeveloped film, put a drop of Bath 3 on the emulsion side, wait for it to clear to a spot, then immerse the piece and time how long the rest takes to clear. That is your fresh clearing time and it is the single most valuable number in this session. Repeat for Baths 1 and 2 on fresh scraps.
23. Write the label for each bottle: what it is, the date mixed, the water used, the volume, and the fresh clearing time. Follow the labelling procedure.
24. Write the batch record. The formula version record worksheet carries the fields; it was designed for developers and works unchanged here.

Expected observations

The beaker goes cold. Dissolving the pentahydrate is endothermic — the supplier’s own safety data sheet says so in a sentence that also draws the contrast: “with anhydrous material, the reaction is exothermic and the solution will retain heat; with the hydrate (crystal) material, the reaction is endothermic and the solution will cool.” Expect the temperature you wrote down at step 1 to have fallen noticeably by step 3. This is why Reilly instructs starting with water slightly warmer than the working temperature, and it is why the anhydrous salt behaves oppositely, which is a good demonstration that a hydrate and its anhydrous form are different substances rather than different labels.

Bath 2 stays clear if the order was right. A faint haze that clears on standing is dissolution finishing. A persistent milkiness is not.

The demonstration goes milky. Not instantly, and not dramatically: a faint opalescence within a minute that deepens over ten into a pale yellow cloudiness that settles very slowly. Kodak’s 1928 troubleshooting section describes exactly that and distinguishes it from the other sludge: sulfur is pale yellow and settles slowly; aluminium sulfite is white and gelatinous. You will have made the first and, if the session goes wrong, may see the second.

The pH readings. The only published figure the course holds for a comparable product is ILFORD’s, for its own non-hardening ammonium fixer: pH 5.0 to 5.5 at both 1+4 and 1+9. Bath 3 should land there, and if it does not, suspect the meter before the fixer. The course holds no published pH for F-5 or for a plain hypo bath, so Baths 1 and 2 have nothing to check against; what you can state is the ordering — the plain bath least acid of the three, the acid bath most.

What is happening chemically

Three things, and each of them was designed into the order.

Dissolution, and where the heat goes. Breaking the crystal lattice costs energy and hydrating the ions returns it, and for the pentahydrate the first is larger, so the solution cools. The anhydrous salt has less lattice to break relative to the hydration it gains, and warms. Nothing about the chemistry of fixing depends on this; the reason it is a step in the procedure is that it changes the temperature at which the alum goes in, and the alum is the ingredient that minds.

Why sulfite goes in before acid. The acid, meeting thiosulfate, makes thiosulfuric acid, which falls apart.

H₂S₂O₃ → H₂SO₃ + S

Sulfurisation: what the demonstration is showing you

Sulfite reverses it, because the equilibrium runs both ways and sulfite is on the other side.

Na₂SO₃ + S → Na₂S₂O₃

Which is also how hypo is manufactured

So the sulfite is not merely a preservative against aerial oxidation; it is a reservoir sitting on the product side of the reaction the acid is trying to drive. Put it in first and the acid arrives to find the escape route already blocked. Put the acid in first — which is step 15 — and there is nothing to block it.

Why alum goes in last, cold. Aluminium(III) in water hydrolyses, which is what makes an alum solution weakly acid, and it stays in solution only over a limited pH range and at a limited temperature. Added to a bath whose acid is already in place and whose temperature has come back down, it dissolves and stays dissolved. Added early, or warm, or fast, it meets sulfite in the wrong conditions and precipitates as the white gelatinous aluminium sulfite the 1928 primer describes.

Data to record

• Water used, and its source, for each bath.
• Temperature before and after dissolving each thiosulfate charge, and the difference.
• Every mass actually weighed, to the readability of the balance, alongside the mass the formula asked for.
• The acid product’s stated strength, the mass weighed, and the acetic acid it therefore contains.
• pH of all three baths, with the meter’s calibration slope and the time of calibration.
• Specific gravity of Bath 3 at 1+4 and 1+9, if you have a hydrometer.
• The fresh clearing time for each bath, on a named film, with the temperature of the bath.
• Observations of the demonstration at 30 seconds, 2 minutes and 10 minutes.
• The label text, copied into the notebook, so the notebook survives the bottle.

📓The capacity sheet, which starts today

A fixer’s label needs one thing a developer’s does not: a running total. Rule three columns on the back of the batch record, or on a card taped to the bottle, and fill one line per session.

Date · what went through it, in films or in 8 × 10 equivalents · running total · clearing time if measured

Two conventions make it work. Count a 120 roll and a 135-36 roll as one film each and four 5 × 4 sheets as one, which is close enough for a log and much better than not counting. And measure the clearing time every time the running total passes a quarter of the published capacity, so that four measurements bracket the corner rather than one measurement finding it too late.

The course has no printed worksheet for this yet, and says so rather than pretending: the formula version record carries the batch fields but not the running total, so the capacity sheet is three ruled columns you draw yourself.

Analysis

1. Compare your weighed masses with the formula’s, as a percentage. A 240 g weighing at ±1 g is 0.4 per cent and a 15 g weighing at ±0.1 g is 0.7 per cent — the small weighings are the less accurate ones, which is the opposite of most people’s intuition.
2. Convert your acid weighing into mol/L and compare with F-5’s 0.296 mol/L. If you used a product whose strength is given by volume rather than by mass, say what assumption you had to make and how much it could be worth.
3. Rank the three baths by pH and say whether the ranking is what the ingredients predict.
4. Compare Bath 3’s pH and specific gravity with the maker’s published figures. A disagreement is information: too low an SG means the bath is dilute, which ILFORD says reduces efficiency, and the first lesson of this part shows costs capacity proportionally.
5. Rank the three baths by fresh clearing time and predict, before the experiment, which will hold out longest under load.

Troubleshooting

What you see	Most likely cause	What to do
Bath 2 is milky and pale yellow	Acid met thiosulfate without enough sulfite — the order slipped, or the sulfite was not fully dissolved	Kodak’s 1928 answer is blunt: throw the bath away and mix fresh. The sulfur will not redissolve at room temperature
Bath 2 has a white gelatinous sludge	Too little acid, or alum added warm, or alum added before the acid	Mix fresh, checking the acid arithmetic first: a 28 % product used where the formula assumed glacial is less than a third of the acid intended
Bath 2 will not clear after standing	Undissolved solid rather than a precipitate	Warm gently and stir; if it clears on warming and returns on cooling, it is alum coming out and the bath was over-charged or under-acidified
The hypo will not dissolve	The water was too cold, and it is getting colder as it dissolves	Add warmer water rather than more time. This is the endothermic effect in the way
The pH meter reads something implausible	The meter, nine times in ten	Re-calibrate at 4.01 and 7.00, rinse the electrode, and read again. A fixer is a high-ionic-strength solution and unkind to a tired electrode
Bath 3 has an obvious layer or a schlieren shimmer	Concentrate and water not properly mixed	Stir hard for a full minute; ILFORD warns that fixer concentrates do not readily mix with water
The concentrate smells of ammonia	The bottle has gone alkaline, or it is not what the label says	Do not use it. An ammonium fixer above about pH 8 evolves ammonia, and the smell is a pH reading you did not intend to take

Clean-up

Rinse every vessel and the stirring rod into the rinse container before anything dries; dried fixer is a white bloom that will contaminate the next thing the vessel touches. Wash the pH electrode as Part III’s page requires and store it wet. Wipe the balance pan. Put the covered demonstration beaker’s contents into the acid container. Gloves off last, and hands washed.

Storage

Cool, dark, capped, upright, and not on the same shelf as an acid — thiosulfate and acid are a pair on the incompatibility matrix, and shelf adjacency is how a spill becomes a reaction.

ILFORD’s keeping figures for working-strength solutions of its own fixer are the best guide the course has for any of these: six months in a full tightly capped bottle, two months in a tank or dish with a floating lid, one month in a half-full bottle, and seven days in an open dish. Fill to the neck, and if you decant, decant into a smaller bottle rather than leaving a large one half empty.

The plain bath is the one to use first. Reilly records that dissolved thiosulfate decomposes in part to sulfite and sulfur on standing, and the plain bath is the one with no sulfite in it to work against that.

Disposal considerations

Nothing mixed today is waste today. What the session produced is about 21 mL of sulfurised, acid thiosulfate and some rinse water.

The chemistry. Acidified thiosulfate carries colloidal sulfur and bisulfite, and further acidification will release sulfur dioxide; the supplier’s safety data sheet names sulfur dioxide and hydrogen sulfide as what it makes with acids. That is why the sample goes to the acid container rather than to any container holding a sulfite or thiosulfate stream, and why it must not go to a container holding a sulfide. Rinse water from this session carries thiosulfate, sulfite and aluminium at trace concentrations and no silver at all, which is the one respect in which this session’s waste differs from every other fixer page in the course.

General practice. ILFORD’s guidance for domestic users in the United Kingdom is to bottle photographic chemistry separately, label it, and take it to a household waste and recycling centre. The disposal page sets out how the course reads the classification and why it computes no threshold.

The chemistry above is general and does not change; what may lawfully be done with a bottle of it does. Check your local regulations, which govern, which differ between authorities inside one country, and which change — the disposal page explains why the course states a caveat here rather than an instruction.

Questions

1. You have only the anhydrous thiosulfate. How much replaces F-5’s 240 g of crystals, and what is the percentage error if you weigh 240 g of the anhydrous salt by mistake?
2. Your acid is labelled “80 % acetic acid” with no basis stated. What is the worst case if it is 80 % by volume rather than by mass, and does that make the bath too strong or too weak?
3. The demonstration used the plain bath rather than the acid bath. Why is that the right choice, and what would you expect if you repeated it on Bath 2?
4. Bath 2 comes out at a pH well below Bath 3’s. Name two things that could produce that and say which measurement would tell them apart.
5. Kodak Limited’s handbook gives F-5 a useful life of 120 sheets of 8 × 10 per 160 fluid ounces. Convert that to sheets per litre, and say why it is not directly comparable with ILFORD’s figure of 40 fibre-base sheets per litre.

Further experiments

Compare complete F-5 with a separately documented variant. The standard bath above retains Kodak’s boric acid. A study of the buffer’s effect would require a second, clearly labelled formula version, a stated hypothesis, an unchanged processing load and the same Level B controls for the reference bath. Record what changes and why; never label the modified bath simply F-5. The course holds no measured side-by-side sludging-life result, so predict a direction and test it rather than writing a numerical advantage into the notebook before measuring.

Make the same bath from a hardener stock. Kodak’s F-53 is the acid, sulfite and alum pre-combined and F-54a is that stock plus hypo. Mixing it that way teaches the instruction the 1949 handbook attaches — the hardener goes into the hypo solution slowly, with vigorous stirring, and both solutions must be cold — and it is the route a working darkroom used, because one stock served several baths.

Push the demonstration in the other direction. Repeat step 15 on Bath 2, adding acid a millilitre at a time and recording the volume at first opalescence. That is a crude titration of the sulfite’s protective capacity, and comparing it with the sulfite you weighed in is the experimental version of the argument for sulfite.

Measure the cooling properly. Repeat step 2 with a thermometer in the beaker and a reading every thirty seconds, for the pentahydrate and for the anhydrous salt at the same molar quantity in the same water. The two curves go opposite ways. The course holds no enthalpy of solution for either salt, so there is nothing published to check your figure against — which makes it a measurement worth keeping.

Check your understanding

Question 1. F-5 calls for 240 g of sodium thiosulfate pentahydrate. You have only the anhydrous salt. How much do you weigh, and what happens if you weigh 240 g of it instead?

• 240 g either way, since it is the same salt
• 153 g; weighing 240 g of the anhydrous salt puts 57 per cent too much thiosulfate in the bath
• 377 g; weighing 240 g would make the bath too weak by a third
• 120 g; weighing 240 g would double the strength

Show the answer and why

Answer: 153 g; weighing 240 g of the anhydrous salt puts 57 per cent too much thiosulfate in the bath

The pentahydrate is 248.19 against 158.11 for the anhydrous salt, so 240 × 158.11 ÷ 248.19 = 152.9 g, and Kodak's own alternative is 150 g — a two per cent rounding that is itself a useful check on your arithmetic. Weigh 240 g of the anhydrous salt and you have 240 ÷ 152.9 = 1.57 times the intended thiosulfate. That is not a disaster in a fixer, which is forgiving of strength in a way a developer is not, but it is 57 per cent more salt than you paid for per litre, and it takes the bath past the 30 to 40 per cent range Kodak says fixes most rapidly.

Question 2. In the demonstration, 1 mL of dilute acetic acid is added to 20 mL of plain hypo and the beaker goes faintly milky. Which sludge is that, how would you tell it from the other one, and why does Bath 2 not do it?

• Aluminium sulfite; it is white and gelatinous; Bath 2 does not do it because it contains no alum
• Free sulfur; it is pale yellow and settles very slowly, where aluminium sulfite is white and gelatinous; Bath 2 does not do it because its sulfite sits on the other side of the same equilibrium and pushes the sulfur back
• Silver sulfide; it is brown; Bath 2 does not do it because it contains no silver
• Undissolved thiosulfate; it clears on warming; Bath 2 does not do it because it is warmer

Show the answer and why

Answer: Free sulfur; it is pale yellow and settles very slowly, where aluminium sulfite is white and gelatinous; Bath 2 does not do it because its sulfite sits on the other side of the same equilibrium and pushes the sulfur back

Kodak's 1928 troubleshooting section distinguishes the two by appearance and by cause: pale yellow and slow-settling is sulfur, from too much acid, too little or impure sulfite, or high temperature; white and gelatinous is aluminium sulfite, from too little acid or too little hardener. The demonstration uses the plain bath precisely because it has no sulfite, which isolates the variable. And the primer adds the consequence that makes this more than a curiosity: if a sulfurised bath is used, the sulfur is apt to penetrate the gelatin and may later cause fading.

Question 3. Why is the potassium alum added last and to a bath at room temperature, when the acid could just as easily go in after it?

• Because alum dissolves faster in a cold solution
• Because the acid is what keeps the aluminium in solution, and warmth and the wrong order both throw it out as aluminium sulfite — the 1949 handbook makes cold and slow explicit for a separately mixed hardener
• Because alum reacts with thiosulfate and must be kept away from it
• Because the alum would otherwise neutralise the acid

Show the answer and why

Answer: Because the acid is what keeps the aluminium in solution, and warmth and the wrong order both throw it out as aluminium sulfite — the 1949 handbook makes cold and slow explicit for a separately mixed hardener

Aluminium(III) hydrolyses in water and stays dissolved only over a limited pH range, so it needs the acid already present. Kodak Limited's handbook states the order for exactly this reason — "if decomposition of the hypo and precipitation of the alum are to be avoided" — and where the hardener is a separate stock it adds two more conditions: slowly, with vigorous stirring, and both solutions cold. Note that alum and thiosulfate are not simply incompatible: their reaction is the hypo-alum sepia toner, run deliberately at 49 to 52 °C, which is the same chemistry used on purpose.

Question 4. Your rapid fixer at 1+4 reads pH 5.2 and a specific gravity of 1.045 at 20 °C, against ILFORD's published 5.0 to 5.5 and 1.070 to 1.080. What has happened and what does it cost?

• Nothing; SG is not a meaningful measurement on a fixer
• The bath is under-strength — the pH is right because pH does not depend much on dilution here, but the SG says the concentrate is short, and capacity falls in proportion to the thiosulfate that is not there
• The bath is over-strength, since a lower SG means more concentrate
• The meter needs recalibration, since a correct pH and a wrong SG are impossible

Show the answer and why

Answer: The bath is under-strength — the pH is right because pH does not depend much on dilution here, but the SG says the concentrate is short, and capacity falls in proportion to the thiosulfate that is not there

Specific gravity tracks how much dissolved solid is in the bottle; pH tracks the buffer, which a dilution moves very little. So the pair diagnose exactly this fault, and it is why ILFORD publishes both. 1.045 sits between the published figures for 1+4 and 1+9, which points at something like a 1+7 by accident — a 100 mL cylinder read as 200, or water added to the wrong line. The cost is capacity, and it is proportional: the equilibrium ceiling is essentially one silver per two thiosulfate, so a bath with three quarters of the thiosulfate takes three quarters of the films, and no extra time recovers it.

Question 5. This session produces almost no waste, and the page makes a point of saying so. What is the point being made?

• That a freshly mixed fixer needs no controls, since it has not been used
• That the hazard and the waste value of a fixer belong to what it has dissolved rather than to what it was mixed from — these three litres carry no silver, and they become a silver-bearing stream the moment they meet a film
• That mixing from scratch is more environmentally responsible than buying concentrate
• That the demonstration is the only hazardous step, so the rest can be done without controls

Show the answer and why

Answer: That the hazard and the waste value of a fixer belong to what it has dissolved rather than to what it was mixed from — these three litres carry no silver, and they become a silver-bearing stream the moment they meet a film

A fixer is a reagent before it is a waste. The reason spent fixer is collected, labelled and kept out of the drain is the dissolved silver it carries and the oxygen demand of the thiosulfate that carried it, and on this bench neither has happened yet. Recognising that is what lets the reader sort a stream correctly rather than by the name on the bottle — which is the judgement the general waste procedure asks for in its first step. Note what does not follow: the controls stay on all session, because the sulfite, the acid and the alum have their own reasons, and the demonstration has its own.

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